In modern applications such as aerospace applications, cylinder head manufactures, engines etc., large integral aluminium alloy structural components are widely used. The main requirement of such aluminum alloys is that their properties should be good at high temperatures ranging from about 200° C. to 300° C. as well as at room temperature.
Aluminium alloys of 2XXX series with high strength at room temperature are commercially available and are extensively used in aerospace applications. For example, alloys such as 2034 are used for fuselage skin, bulk heads wing lower skin, stringers and panels as well as in ribs and spars. Another commercially available alloy is 2024 that is widely used for high strength and high toughness applications. These alloys have yield strength of about 395 MPa with 10% elongation at room temperature, but are not suitable for high temperature applications.
Among aluminium alloys, 2219 alloy possesses high strength at elevated temperature. Major applications of this alloy are in aerospace industries. However, the application of this alloy is also restricted to a maximum temperature of 150° C., above which, the strengthening precipitates coarsen rapidly resulting in steep loss in strength. This alloy in T8 temper has yield strength of about 355 MPa with 9.4% elongation at room temperature, while at 250° C., it is about 159 MPa with 21% elongation. Therefore, aluminium alloys with good strength at temperatures above 150° C. pose a challenge.
Heat treatment plays a crucial role in tuning mechanical and physical properties of aluminium alloys. Conventionally, these alloys are processed through solution heat treatment, quenching followed by aging (natural or artificial). Additionally, cold working is also occasionally introduced prior to aging. The solution treatment is done to dissolve the alloying elements into solid solution of the matrix. After this treatment, the aluminium alloy is quenched to room temperature to retain the alloying elements in aluminium solid solution termed as supersaturated solid solution. This alloy is then heated to intermediate temperature and held (aging) so that the supersaturated solid solution is decomposed to form finely dispersed precipitates in aluminium matrix. The decomposition of the solid solution involves the formation of Guinier-Preston (GP) zones and metastable intermediate precipitates. The GP zones are solute rich clusters of atoms and are coherent with the matrix. Metastable intermediate precipitates are normally larger in size than GP zones and are partly or fully coherent with the lattice planes of the matrix. This phase may form homogenously or may nucleate heterogeneously on GP zones or lattice defects such as dislocations. Mechanical deformation prior to aging increases dislocation density and provides more sites where heterogeneous nucleation of intermediate precipitates may occur. The strengthening of the alloy occurs due to the presence of these precipitates by several mechanisms which have been reported in the literature.
The heat treatment temperatures depend on the alloy system. For example, for 2XXX series alloys the solutionizing temperature is between 530° C.-540° C. and aging temperature is between 130° C.-200° C. For 7XXX series alloys, the solutionizing temperature is between 450° C.-470° C. and the aging temperature is 120° C.-135° C. For some alloys, duplex aging (two stage aging) is carried out. For example, 7075 alloy is processed through duplex aging in which the first stage at low temperature (121° C.) involves precipitation of GP zones and in the second stage at slightly higher temperature (171° C.) metastable intermediate n′ precipitates form. In this case, 1st stage gives pronounced hardening while 2nd stage results in a significant improvement in stress corrosion cracking.
U.S. Pat. No. 6,074,498 discloses Al—Cu—Li—Sc alloys produced by duplex aging. After solutionizing and quenching, the alloy is aged between 120° C.-140° C. for 8 to 30 hrs followed by aging at temperatures between 150° C.-170° C. The final microstructure contains metastable θ′ (Al2Cu) and δ′ (Al3Li) precipitates throughout the aluminium matrix. This alloy gives good room temperature strength but at temperatures above 150° C., these precipitates coarsen rapidly resulting in low strength.
In recent past, for improving high temperature (>200° C.) stability of aluminium alloys, transition metals (TM) such as Sc, Zr, Ti, Hf etc. have been added in small amounts. These elements form trialuminides (Al3TM) with aluminium at Al-rich portion of the phase diagram. Most of them have DO22 or DO23 equilibrium structure but they can also form metastable L12 structure, which is coherent and uniformly distributed in the aluminium matrix. These dispersions have high melting points and are stable at high temperature up-to 300° C. due to very low diffusivity of these transition metals in aluminium. These alloying elements often have very low solubility in aluminium and hence in order to achieve higher super saturation in aluminium a high cooling rate during solidification from the liquid melt is often required. After super saturation, these can be aged at intermediate temperature between 300° C.-400° C. so that it will decompose to yield nanometric coherent L12 type precipitates in aluminium matrix. In normal casting route, high super saturation cannot be achieved. Several investigators reported decomposition of chill cast Al-TM alloys to L12 Al3TM coherent dispersions in aluminium matrix. Their room temperature strengths are relatively low as compared to the existing aluminium alloys. However, their stability at high temperatures is better than the existing aluminium alloys.
U.S. Pat. No. 6,248,453 discloses Al alloys with improved strength up to 300° C. by dispersion of fine L12 intermetallics Al3X, where X is Sc, Er, Lu, Yb, Tm or U. Since the dispersion of these fine intermetallic particles necessitated high cooling rates (104-108 K/s), it was achieved by employing rapid solidification techniques, such as, gas atomization and melt spinning. Such rapid solidification techniques generally produce powder, fibres or ribbons, which necessitated consolidation.
In light of foregoing discussion, it is desirable to produce an aluminium alloy with higher strength at both room temperatures as well as at high temperatures, and a method of producing such aluminum alloy by casting, rather than rapid solidification techniques, to overcome the limitations stated above.